Does anyone have an idea of how a fork spring rate might change after 10 years of on and off use? I don't think it is unreasonable to think that a .90 spring out of the box that get used and handed around from racer to racer over the years becomes a .8x? The question is by how much does it change on average?
I rebuilt the forks on my 2003 RC51, 30 some thousand miles on the clock. The stock rate is 1.0. Measuring the springs (free length, wire diameter, # of active coils) and calculating the spring rate, I came up with .98. So not much.
Stock forks came with negative preload on that bike, IIRC. No real constant load on those springs might have helped them live longer!
Calculating the spring rate won't show changes due to fatigue. The steel is actually getting weaker, not changing dimensions. You'd need to measure the spring rate.
I haven't looked at any of my old textbooks in years, but wouldn't the fatigue cause the local structure to venture into plastic deformation and shorten the spring over time? Hence shy you measure clutch and valve springs to make sure they're still in spec? AKA, the spring will "sag." I wouldn't think fork springs would be at a high risk of sag, just based on coil count/length/wire diameter. Unless I suppose you were running an excessive amount of preload and smashing them into or near coil bind all the time. In the RC's case, the long ass top out spring effectively acts like a double stacked spring setup, so the first third of the fork travel has something like a .8 rate then transitions to a 1.0. The top out spring is pushing in the opposite direction of the main springs. At least it did, until I took it out and put a normal top-out spring in it and new springs.
The top out spring works in the opposite direction as the main spring. So the top of the fork travel acts to compress the main spring and extend the top out. Once the top out spring is fully extended, only the main spring is engaged. I gather the theory was to provide a softer fork when the weight was being transferred rearward under acceleration and a firmer rate under braking. It makes the preload adjuster super weird, as it basically just compresses the top out spring.
Theory is to keep front wheel on the ground and suspension working under acceleration. If force is pushing wheel and bump acting against then rate is both springs together. It is actually stiffer not softer even if it does not sound right.